Smooth Tone Clickless CW Sidetone Generator

INTRODUCTION

This circuit is about as good as it gets for generating
morse code tones. It may be used as a code practice oscillator,
a tone generator for a keyer, a sidetone oscillator for a CW transmitter
or an audio Modulated CW (MCW) generator for an FM transmitter or repeater.

Feng shuei is the Chinese art of arranging objects in buildings, it is based
on many practical ideas. One important concept of feng shuei is the rounding
of sharp corners so that you don't scrape yourself when walking by.
Think of this circuit as a form of electronic audio feng shuei ;-).

One may think this is a lot of circuitry
just to make simple beep tones, but the real value of the circuit is
that it sounds very smooth when compared to a clicky and harsh-toned
square wave code oscillator.
This circuit is easy on your ears, even with extended use.

There are other applications for a clickless beeper, one reader
was interested in cranking the frequency into the ultrasonic range
for use as a dog training device.

The tone quality sounds just like a well shaped CW signal from the airwaves,
minus the background noise. Morse code is much easier to listen to when the
waveform is smooth, both the audio tone and the tone's envelope are smooth.

Careful readers may notice the lack of a pitch control, this is mainly
to keep the circuit simple.
The components in the circuit shown produce a 500 hz tone.
Changing the pitch requires the use of
different part values on both the oscillator and low pass filter sections.

For a different pitch, change the value of the capacitor between
pins 13 and 14 of the LM324, and change the two 33K resistors on pins
14 and 9 of the LM324, both resistors should be the same value.
An easy, empirical way to find the filter resistor values is to wire
in a ganged dual 100K potentiometer in place of the two resistors,
tune until a good sine wave is achieved, and measure the resistance on
the potentiometer. Substitute the nearest fixed value resistors into
the circuit, or just permanently connect the dual potentiometer
for a tuneable low pass circuit.

A three pole, multi-position switch can be used
to select a number of different tones and filter settings. Each tone
requires a specific oscillator capacitor and pair of equal-value filter
resistors.

This is revision two of the circuit, changes include the addition of
the LM386 audio amplifier IC for greater audio volume, and electrical
isolation of the keying input via the 4N35 opto-isolator. The isolation
makes the circuit more immune to damage from static electricity zaps on
the key.

To further purify the sine wave tone, you can mount
a speaker in a tube and cut the tube so that it resonates at around 500 hz
with the speaker in place.
The resonant tube will also increase the volume.

If the circuit is to be used as a transceiver sidetone generator, remove
the LM386 IC and feed the signal from the volume pot into the
transceiver's audio circuitry.

This circuit will work over a supply voltage range of 9V to 14V.
It could be run from a 9V battery, although the 18mA idle current
would produce a fairly short battery life.

Theory

This circuit is essentially a minimal version of an analog music synthesizer
that is hard-wired for the purpose of making morse code tones.

The oscillator section produces a fixed frequency triangle wave on LM324
pin 14, that is fed into a low pass filter to get a sine wave on LM324 pin 8.
A triangle wave can be thought of as a series of sine waves with decreasing
amplitudes as the frequency increases. The low
pass filter simply removes the upper harmonics and passes the
fundamental sine wave frequency through.

The sine wave is sent to a voltage controlled amplifier made from three
2N3904 transistors. The upper two 2N3904 transistors should be a
matched-gain pair in order to minimize thumping sounds.
The VCA is modulated with a keying waveform to produce the resulting
modulated waveform.
The keying waveform is generated by smoothing out the
square wave keyed signal on the 4N32 emitter with the 220K/220K/20n (.02uF)
capacitor/resistor low pass filter on the base of the lower transistor.
The two 220K resistors in series with the 4N35 output transistor act as
a voltage divider, this causes the the keying envelope to swing between
0V and 6V, which is a good range for controlling the VCA.

Keying of the circuit is performed either by the manual key input, or
via the keyer input, which can be used to connect the circut to a
morse code keying chip. To use the keying input, an active low open
collector driver should be used.

The audio signal comes out of LM324 pin 7, it connects to a 10K volume
potentiometer and an attenuator made with two resistors.
The attenuated audio is then fed to the LM386 audio amplifier.

The 200n capacitor on the audio output prevents the LM386 from generating
RF oscillations, which it likes to do. I originally tried using
the series R-C network that is normally shown on LM386 application
notes, but the LM386 would break into RF oscillations.

The circuit nodes marked +12Vf are all connected together, this is the
filtered +12V line. The +12V line is filtered through a 10 ohm resistor
and across a 100uF capacitor, those components remove audio noise
that may be present on the +12V supply.

A careful observer will notice that there are two identical +6V divider
chains, one for the oscillator and one for the VCA.
I started with a single divider, but the oscillator tone changed when the
circuit was keyed. An op-amp divider could also be used, but it would cost
more and consume more current.

Construction

The Sidetone Generator was built on a
OnePas
brand prototype circuit board.
The wiring pattern on this board is the same as that found on
many wireless prototype socket boards. This makes it possible to
build a temporary circuit on the plug board and transfer it to a
more permanent layout on the OnePas board.
It should be fairly easy to copy this circuit layout just by looking
at the photograph, there are a few trace cuts and insulated jumpers
on the bottom of the board. Also, a few extra holes were drilled in
the board to hold the potentiometer.

David McClafferty, VE1ADH, has provided the following
ExpressPCB circuit board files:

Paul Christensen (W9AC) produced a modified version of this circuit:
CW-Sidetone-Generator.pdf (65K)
that features a tuneable oscillator frequency and reduced key up
feedthrough. The feedthrough in my circuit is inaudible as long as the two
upper VCA transistors have matched gains. Modern transistors from the
same manufacturing batch are usually sufficiently matched for this circuit.